In order to understand the biological and functional relationship between mammographic density and breast[unreadable] cancer risk, experimental models are needed to investigate key features in a controlled context. We propose[unreadable] that the mechanistic link between dense breasts and breast cancer risk is that the biological processes that[unreadable] lead to increased breast density can promote malignant progression by initiated cells in the adjacent[unreadable] epithelium. We hypothesize that features of increased stroma, remodeled extracellular matrix (ECM) and[unreadable] molecular markers in mammographically dense breasts are indicative of an 'activated' stroma. Part of the[unreadable] normal continuum of stromal phenotypes, activated stroma has similarities to the stromas formed during[unreadable] morphogenesis and wound healing. Our experimental studies and those of others have demonstrated that[unreadable] atypical stroma can become an active participant in cancer progression. Project 4 will develop two[unreadable] activated stroma (AS) mouse models to test these hypotheses. An exogenous agent, ionizing radiation (IR),[unreadable] and a genetically engineered mouse expressing constitutively active transforming growth factor beta1 (TGF-beta)[unreadable] on a fibroblast-specific promoter (FSP-Tgf-beta (223-225)) will be used to generate mammary AS. Radiotherapy for[unreadable] breast cancer transiently increases mammographic density. We have shown that IR induces mammary[unreadable] stromal ECM remodeling in mice similar in character to that found in dense breast tissue. Together these[unreadable] observations provide the rationale that IR may be used as an experimental tool to induce a mouse mammary[unreadable] stroma similar to that in dense breasts. We have shown that TGF-beta1 mediates radiation-induced ECM[unreadable] remodeling and is involved in ovarian hormone regulation of normal breast development. Others have[unreadable] demonstrated that TGF-beta is deregulated during cancer progression. Certain Tgf-beta1 gene polymorphisms are[unreadable] associated with increased breast cancer risk. Thus, TGF-beta fulfills the criteria of being a genetic trait that[unreadable] predisposes certain women to breast cancer as well as a hormonally regulated growth factor whose actions[unreadable] affect mammary tissue composition. The mammary AS produced by IR and the FSP-Tgf-beta(223-225) transgenic[unreadable] mouse models will be compared to the features of dense breast tissue determined in Project 2. We will use[unreadable] these models to answer the following: 1. How do the known breast density modulators of parity, age and[unreadable] hormonal status affect the specific features of AS; 2. Is epithelial proliferation, apoptosis, or morphogenesis[unreadable] affected by stromal activation; and 3. Does AS promote neoplastic progression? We will evaluate markers[unreadable] that are identified in Project 2 and tested in Project 3 in the mouse models to define commonalities that can be[unreadable] used as benchmarks of critical processes. Mouse models permit evaluation of how important lifestyle[unreadable] variables contribute to the expression of a given marker.[unreadable]